In recent years, aspheric surfaces that are not symmetric about an optical axis, i.e., “free-shaped surfaces”, are beginning to be used. A free-shaped surface has freedom in terms of layout and freedom in terms of aberration correction unlike rotationally symmetric optical systems, and is therefore beneficial in making it possible to obtain a small and high-performance optical system. In particular, techniques relating to prisms having a free-shaped surface make it possible, with the progress in material and molding techniques for injection-molded glass and/or resin, to obtain a highly accurate shape, and therefore have a high potential for achieving a small, high-spec and high-performance optical system. However, the light-separating performance of the prism may result in chromatic aberration in the optical system.
Patent Document 1 discloses an optical system in which a diffractive optical element (DOE) is arranged between an eccentric prism having a free-shaped surface and the entrance pupil in order to correct chromatic aberration caused by the eccentric prism, whereby it is possible to correct residual chromatic aberration remaining in the unitary eccentric prism.